257
8 Fishery and Biological Implications of Fishing Spawning Aggregations…
locations where gatherings of ripe fi sh occur may provide selective benefi ts related
to larval dispersal, conditions associated with egg/larval survival, or habitat factors
such as appropriate substrate for nesting (e.g. in triggerfi sh, Chap. 12.23 ) or high
substrate complexity where large numbers of adults can temporarily fi nd shelter.
However, the signifi cance of mate choice or of specifi c habitats for reproducing in
large groups has been little examined in fi shes (Chap. 3 ). Therefore, we must keep
in mind the various possible adaptive reasons for aggregation-formation when
exploring the possible direct and indirect biological impact(s) of exploiting spawning
aggregations.
When fi sheries heavily target spawning aggregations, the most obvious sign of
change to the fi shery manager is likely to be declines in landings volumes or reduced
numbers of fi sh at the aggregation site. Depending on the level of fi shing effort, such
declines may take a long time to become apparent because of hyperstability (Fig. 8.6 ,
Chap. 11 ). Yet signifi cant negative changes to the population may occur before
landings decline. Many reef fi shes have complex social and mating systems that
may be disrupted by fi shing activities. Fishing could directly alter population structure, via shifts in adult sex ratio or mean body size, in ways that affect reproduction.
Indirectly, declines in fi sh numbers due to fi shing could affect reproduction by
impairment of visual, olfactory or auditory stimuli, social cue transmission or by
infl uencing other interactions such as courtship or sexual selection that only occur
during brief aggregation periods. For those species that live in deep-water or habitats or are normally solitary, the spawning aggregation may be the only time fi sh
encounter conspecifi cs for mating, or are exposed to the population sex ratio or
other aspect of the demographic profi le (see Chap. 12.7 ).
The possible biological effects of fi shing on aggregations will, therefore, depend
on the way(s) in which fi shing is conducted, the absolute and relative volumes of
fi sh taken, the selectivity of fi shing, differential movements of sexes in and out of
aggregations and the adaptive signifi cance for the species of the aggregating habit
itself. The combination of direct and indirect effects of fi shing could ultimately
infl uence the reproductive output from aggregations with short and long-term implications for the fi shery as well as for the targeted population. These issues are
addressed below and considered in light of fi shery management implications and
future information needs.
8.4.1 Abundance, Sex Ratio, and Body Size
Unmanaged harvesting from fi sh spawning aggregations by commercial, recreational, or even artisanal fi sheries is often associated with declines in aggregation
size (number of fi sh) over time. This is not surprising, given that heavy fi shing can
remove large proportions of aggregations in just a few days (e.g. 10–20%, Nassau
grouper, Sala et al. 2001 ; 20–30%, camoufl age grouper, Rhodes and Sadovy 2002a,
b ) , and that heavy exploitation of aggregations might refl ect high fi shing mortality
in the fi shery in general. In extreme cases, entire aggregations have been removed
8 Fishery and Biological Implications of Fishing Spawning Aggregations…
locations where gatherings of ripe fi sh occur may provide selective benefi ts related
to larval dispersal, conditions associated with egg/larval survival, or habitat factors
such as appropriate substrate for nesting (e.g. in triggerfi sh, Chap. 12.23 ) or high
substrate complexity where large numbers of adults can temporarily fi nd shelter.
However, the signifi cance of mate choice or of specifi c habitats for reproducing in
large groups has been little examined in fi shes (Chap. 3 ). Therefore, we must keep
in mind the various possible adaptive reasons for aggregation-formation when
exploring the possible direct and indirect biological impact(s) of exploiting spawning
aggregations.
When fi sheries heavily target spawning aggregations, the most obvious sign of
change to the fi shery manager is likely to be declines in landings volumes or reduced
numbers of fi sh at the aggregation site. Depending on the level of fi shing effort, such
declines may take a long time to become apparent because of hyperstability (Fig. 8.6 ,
Chap. 11 ). Yet signifi cant negative changes to the population may occur before
landings decline. Many reef fi shes have complex social and mating systems that
may be disrupted by fi shing activities. Fishing could directly alter population structure, via shifts in adult sex ratio or mean body size, in ways that affect reproduction.
Indirectly, declines in fi sh numbers due to fi shing could affect reproduction by
impairment of visual, olfactory or auditory stimuli, social cue transmission or by
infl uencing other interactions such as courtship or sexual selection that only occur
during brief aggregation periods. For those species that live in deep-water or habitats or are normally solitary, the spawning aggregation may be the only time fi sh
encounter conspecifi cs for mating, or are exposed to the population sex ratio or
other aspect of the demographic profi le (see Chap. 12.7 ).
The possible biological effects of fi shing on aggregations will, therefore, depend
on the way(s) in which fi shing is conducted, the absolute and relative volumes of
fi sh taken, the selectivity of fi shing, differential movements of sexes in and out of
aggregations and the adaptive signifi cance for the species of the aggregating habit
itself. The combination of direct and indirect effects of fi shing could ultimately
infl uence the reproductive output from aggregations with short and long-term implications for the fi shery as well as for the targeted population. These issues are
addressed below and considered in light of fi shery management implications and
future information needs.
8.4.1 Abundance, Sex Ratio, and Body Size
Unmanaged harvesting from fi sh spawning aggregations by commercial, recreational, or even artisanal fi sheries is often associated with declines in aggregation
size (number of fi sh) over time. This is not surprising, given that heavy fi shing can
remove large proportions of aggregations in just a few days (e.g. 10–20%, Nassau
grouper, Sala et al. 2001 ; 20–30%, camoufl age grouper, Rhodes and Sadovy 2002a,
b ) , and that heavy exploitation of aggregations might refl ect high fi shing mortality
in the fi shery in general. In extreme cases, entire aggregations have been removed